Inviscid spinning of silicon steel

ABSTRACT

A method is provided for preventing orifice plugging when melt extruding a steel-silicon alloy to form fine diameter wire. This is accomplished by controlling the oxygen potential in the melt above the orifice at a level wherein the activity of silica within the melt is maintained at from 0.3 to unity - the standard state of unit activity being defined as the melt saturated in silica at the concentrations of silicon and oxygen therein and at the melt temperature.

United States Patent [191 Rakestraw INVISCID SPINNING OF SILICON STEEL[75] Inventor: Lawrence F. Rakestraw, Raleigh,

[73] Assignee: Monsanto Company, St, Louis, Mo.

[22] Filed: Aug. 29, 1973 [21] Appl. No.: 392,829

[52] US. C1...l 164/66; 164/82 [51] Int. Cl. B22d 11/12 [58] Field ofSearch 164/66, 82, 89; 264/176 F [56] References Cited UNITED STATESPATENTS 2,907,082 10/1959 Pond 164/66 3,216,076 11/1965 Alber et a1164/82 X IIIIIIIIIIIIIIIIIIflII/III 1 May 20, 1975 Dunn et .al. 164/66 XPrivott et al. 164/66 Primary ExaminerR. Spencer Annear Attorney, Agent,or Firm-Russell E. Weinkauf [57] ABSTRACT A method is provided forpreventing, orifice plugging when melt extruding a steel-silicon alloyto form fine diameter wire. This is accomplished by controlling theoxygen potential in the melt above the orifice at a level wherein theactivity of silica within the melt is maintained at from 0.3 to unitythe standard state of unit activity being defined as the melt saturatedin silica at the concentrations of silicon and oxygen therein and at themelt temperature.

8 Claims, 1 Drawing Figure t IIIIIIIIIIIIJm'IIIIIIIIIIIIIIIIIIIIIIIIlINVISCID SPINNING OF SILICON STEEL BACKGROUND OF THE INVENTION Thisinvention relates to improvements in the method wherein steel alloys aremelt extruded to produce fine diameter wire.

Until quite recently, it was not possible to fabricate filamentarystructures from metals or metal alloys by the method of melt extrusion.The limiting factor was that the melt viscosity of these materials is solow as to be practically negligible. In other words, the melts of metalsand metal alloys are essentially inviscid.

The problem presented by an inviscid melt when at tempting to extrude itto form filaments is that the surface tension of the filamentary jet, asit issues from the shaping die, is so great in relation to its viscositythat the molten stream breaks up before sufficient heat can betransferred for conversion to the solid state.

This intractable problem has now yielded to a unique solution asdescribed in U.S. Pat. Nos. 3,216,076 and 3,658,979. In accordancetherewith, the nascent molten jet, as it issues from the shaping die, isbrought into contact with a gas capable of instant reaction with the jetsurface. The result is the formation of a thin film which envelopes thejet surface. This thin film has been found to be capable of holding thejet stream together until sufficient heat can be transferred to effectsolidification. For example, fine diameter wire may be formed fromaluminum by extruding the melt at an appropriate velocity into an oxygenmedium. When the hot jet issuing from the extrusion orifice contacts theoxygencontaining atmosphere, a stable film of melt insoluble aluminumoxide forms almost instantaneously about the peripheral surface of thejet. In essence, a sheath is formed which protects the filamentary jetor stream against surface tension break-up until solidification takesplace.

The oxide of aluminum is a solid which is insoluble in the non-oxidizedmolten metal. This, of course, makes film formation by contact withoxygen below the orifice possible. However, in the instance of ferrousmetals, as for example steel, the iron oxide is soluble in the liquidmelt. Consequently, a film will not form when a molten jet is extrudedinto an oxidizing atmosphere.

A solution to this problem is provided in the teachings of U.S. Pat. No.3,216,076. As disclosed therein, filamentary structures may be formedfrom metals whose oxides are soluble in the non-oxidized molten metal byalloying them with a minor percentage of a compatible metal whose oxideis insoluble in the non oxidized molten metal. By compatible metal thereis meant a metal or combination of metals having the ability to form analloy. According to U.S. Pat. No. 3,216,076 metals which may be used forthis purpose include aluminum, magnesium, beryllium, chromium, lanthanumand combinations thereof. The particular metal employed should bepresent in amounts in excess of 0.5% by weight of the alloy. The upperlimit on the quantity of metal which will produce a stable oxide is onlydetermined by the physical characteristics desired in the ultimatefilamentary product. The metal most commonly alloyed with steel foreffecting film forma tion when extruding steel melts has been aluminum.

The extension of the capability for producing filaments directly fromthe melt to metals like steel constitutes an important advance in theart. However, commercial scale practice of this potentially attractivemethod for producing steel wire has been inhibited by an inability tocontrol the tendency for the orifice to plug during extrusion. Oxidationreactions occurring in the melt prior to extrusion are largelyresponsible for the partial or complete plugging of the orifice.Contributing to this problem has been a premature oxidation of thesecond metal used to stabilize the molten stream of steel. As has beennoted, aluminum is commonly used for this purpose, and it has been foundmost diffi cult to maintain the melt oxygen content at the very lowlevels required for avoiding premature alumina precipitation and theformation of solid inclusions in the melt which tend to accumulate inthe orifice area. Likewise, a similar problem exists with other metalsheretofore proposed for alloying with steel to provide a stabilizationcapability.

It is therefore a principal object of this invention to provide a meansfor extruding a steel alloy melt to form fine diameter wire in theabsence of orifice plugging caused by insoluble oxide formation abovethe extru sion orifice.

It is another object of this invention to provide a means forcontrolling the oxygen potential of the melt upstream from the extrusionorifice during the extrusion of molten steel to prevent the occurrenceof orifice plugging.

Other objects and advantages will become apparent from a description ofthe invention which follows:

SUMMARY The above objects are achieved when steel alloy melts areextruded in accordance with a procedure which includes: (1) employing amelt of steel alloyed with silicon, with the silicon being present in anamount of at least 0.5 percent by weight of the alloy; (2) maintaining apressurized gas mixture over the melt consisting of an inert gas and anoxygen containing gas; (3) controlling the oxygen potential of the meltupstream from the extrusion orifice at a level wherein the silica withinthe melt will have an activity of from 0.3 to unity such control beingeffected by maintaining the partial pressure of the oxygen containinggas at the appropriate predetermined value; (4) extruding the melt as amolten filamentary stream directly into an oxygencontaining medium ofsufficient oxidizing capacity to cause silica to precipitate and form astabilizing film about the surface of the stream; and (5) cooling thefilm stabilized stream to the solid state.

DESCRIPTION As above noted, the method of this invention is directed tothe production of fine diameter wire from a steel alloy by meltextrusion. For purposes of definition, fine diameter wire may beconsidered as any wire having a diameter of less than about 35 mils. Itis well known, of course, that steel is an alloy of iron and carbon.Generally, the carbon content will be in the range of from about 0.01 to4.30 by weight of the alloy in steels intended for use in the productionof wire products.

According to this invention, steels of the type just described arealloyed with silicon to provide a filmforming component for the meltextrusion procedure. Generally, the silicon concentration will rangefrom between about 0.5 and 5.0 percent on the total weight of the alloy,although there is no process criticality with respect to the upperlimit. That is, the upper limit may be determined merely on the basis ofthe physical characteristics desired in the ultimate product. However,it does appear desirable that the silicon be present in the alloy in anamount of at least 0.5 percent by weight in order to form a stabilizingfilm of the required strength.

The temperatures employed when extruding the melt are critical only tothe extent that they obviously must be at or above the melting point ofthe alloy. Although not required, it is generally good practice to keepthe temperature l20C. above the liquidus temperature of the alloy duringextrusion to provide a margin for any heat loss which might occur.Likewise, the head pressures employed are critical only to the extentthat they must impart a sufficient stream velocity to form an efficientjet in accordance with the parameters as set forth in US. Pat. No.3,658,979.

In the film stabilization of inviscid steel jets according to thisinvention, the viscous film is generated by oxidation of the siliconadded to the steel expressly for that purpose. This is brought about byextruding the siliconcontaining molten jet directly into an oxidizingmedium. Thus, as the jet emerges from the extrusion orifice it isimmediately contacted with an oxidizing atmosphere and a film of silicais caused to form almost instantaneously.

It has now been found that when carrying out melt extrusion operationsin accordance with the procedure as outlined above, the formation oforifice-plugging inclusions can be greatly reduced by maintaining theactivity of silica in the molten mass above the orifice at valuesbetween 0.3 and unity. The standard state of unit activity for thepurposes of this invention is defined as the melt saturated in silica atthe concentration of silicon and oxygen therein and at the temperatureof the melt.

The activity of silica within the melt is controlled by means of anoxygen-containing gas which is introduced into the system with an inertgas to provide a positive gas pressure for effecting extrusion. That is,the partial pressure of the oxygen-containing gas in the gas mixtureprovides the mechanism for this control. The appropriate partialpressure for any given run will, of course, depend upon the particulargas employed, the carbon and silicon concentrations within the melt andthe melt temperature. With these parameters being known for anycontemplated operation, those skilled in the art can readily calculatethe particular partial pressure values which are needed to accomplishthe desired result.

Among the oxygen-containing gases which may be employed are carbonmonoxide, carbon dioxide, oxygen and steam with carbon monoxide havingparticular advantages in practice. However, since the purpose of the gasis to function merely as an oxygen donor to the melt chemistry, thechoice of an oxygen-containing gas is essentially without limitation.Any suitable inert gas may be employed as the second component in thepressurized gas mixture. For example, argon and helium are commonlyemployed.

As previously noted, the oxygen content in the melt above the orificeshould be controlled at a level which will insure a silica activity offrom 0.3 to unity. Generally, best results are realized when the oxygenlevel in the melt is at or relatively near saturation with respect tosilica and the value of the silica activity is from about 0.9 to unity.The reason for this is that the ease of stabilizing silicon-containingsteel jets as they emerge from the extrusion orifice is determined bythe amount of oxygen dissolved in the molten jet. Hence, asiliconcontaining steel melt which is saturated or substantiallysaturated with oxygen, vis-a-vis silica is stabilized with greaterfacility than one which is highly undersaturated in relation to silica.

Substantially higher oxygen levels can be tolerated in steel-siliconmelts because of the relatively high solubility of silica (SiO which isgenerated in the presence of oxygen. That is, the melt solubility ofsilica far exceeds that of alumina (A1 0 or the oxides of other secondmetals previously employed with steel for effecting film-stabilization.For example, the use of aluminum at 1.0 weight per cent as a secondmetal requires that the melt oxygen level be controlled to a value of 4ppm or less in order that precipitation of the oxide be avoided. On theother hand, melt oxygen levels of ppm or more can be tolerated whensilicon is substituted for aluminum at the same concentration. As apractical matter, it is virtually impossible to exercise the controlrequired in the case of aluminum, i.e., to maintain the oxygen level atless than 4 ppm. More over, the use of silicon provides the furtheradvantage in that when the oxide thereof is precipitated from the melt,it forms non-plugging viscous inclusions in contrast to the crystallinesolids characteristic of alumina or other metal oxide precipitates.

When the oxygen potential of the melt is too low, it becomes highlyundersaturated with respect to silica. The result is that the meltchemistry is then dominated by the oxides of melt impurities whosesolubility limits are lower than that of silica. These oxides areusually hard solids which accumulate in the orifice area uponprecipitation and eventually plug it.

As previously noted, film stabilization is brought about by extrudingthe silicon-containing molten jet directly into a gaseous medium havinga sufficient oxidizing capacity for causing silica to precipitate andform a film about the peripheral surface of the jet. Although anoxidizing atmosphere rich in carbon monoxide is generally preferred, anyoxygen-containing gas or gas mixture having sufficient oxygen potentialfor effecting silica formation in the molten stream may be employed. Inaddition to carbon monoxide other suitable examples which may bementioned are carbon dioxide, oxygen, sulfur dioxide and steam. Forpurposes of illustration only, the film stabilization chemistry will bedescribed in terms of a carbon monoxide oxidizing medium. It will beunderstood that other oxygencontaining gases could likewise be employed.The reactions which occur may be set forth as follows:

1. the absorption of gaseous carbon monoxide (CO by the liquid jet togive dissolved carbon (C) and oxygen (0).

ZCOW, 2c 20 followed by;

2. the reaction of silicon in the liquid steel with dissolved oxygen toform a solid silica (SiO film on the jet surface 20 Si 7- SiO 2C Theoverall reaction is, thus, the sum of reactions (1) and (2).

3. ZCO Si r: SiO 2C For the stabilizing film of solid silica to form, itis necessary that the solubility limit of oxygen on the steel jetsurface be exceeded with respect to silica. This is brought about byexceeding the equilibrium partial pressure of carbon monoxide in theoxidizing atmosphere into which the steel jet is extruded. The totalcarbon monoxide pressure required for stabilization may be defined asfollows:

where P is the total CO partial pressure required for stabilization,

P is the equilibrium partial pressure, and

P is the driving force required to form a sufficiently strongstabilizing film within the required time limit.

It is seen from the above discussion that orifice plugging is avoidedand a proper stabilization of the extruded jet is achieved by an abilityto control the oxygen content within the steel-silicon melt at thedesired level both above and below the extrusion orifice. That is,before the steel passes through the orifice the activity of silica inthe steel melt is controlled to a value of between 0.3 to unity, withfrom 0.9 to unity being preferred. As soon as the melt exits from theorifice as a filamentary jet, the oxygen level is increased and a filmof precipitated silica is thereby formed before varicose breakup of themolten jet can take place.

As a final step in the production of fine diameter steel wire inaccordance with this invention, the film stabilized molten stream or jetis passed into a cooling medium to effect solidification. It isdesirable to utilize a gas with good thermal conductivity for thispurpose. That is, gases such as helium, hydrogen, carbon dioxide,nitrogen or mixtures thereof may be suitably em ployed with hydrogen andhelium or mixtures of hydrogen and nitrogen being of particularpreference.

For a description of a representative type apparatus which may beemployed for producing fine diameter wire in accordance with thisinvention, attention is directed to the drawing wherein the singleFIGURE depicts a schematic, partially sectionalized, vertical view of aninduction heated extrusion apparatus.

As shown there, such apparatus is comprised of a crucible 2 having abase plate 3, the crucible and base plate being supported on pedestal 4and enclosed within an insulating cylinder 5 and a susceptor 6 employedin conjunction with induction heating coils 7. The unit is pressurizedby gases brought into the head 9 through conduit 8. Sealing rings 10serve to maintain the pressure within the enclosure by preventingleakage past the base plate. The molten metal 1 is forced throughorifice 11 in orifice plate 12 by the gaseous head pressure and emergesfrom orifice 11 as a cylindrical molten jet 13. The nascent jet passesthrough an oxygen-containing gaseous atmosphere contained within cavity14 provided by the pedestal 4. The oxygen-containing gas is brought intocavity 14 via conduit 15.

It is to be understood that the justdescribed extrusion apparatus ismerely a schematic representation of a typical assembly which may beemployed in the practice of the present invention. Many designvariations are possible and will readily occur to those skilled in theart. For example, all or part of the pressurizing gas mixture could beintroduced into the system by providing a means for bubbling the gasesup through the melt as an alternative or supplementary means to theintroduction above the melt surface as shown in the drawing. Theimportant consideration is that the oxygencontaining gas be provided tothe system at the proper partial pressure. Moreover, a resistance-heatedassembly could be substituted for the illustrated inductionheated unit.The following examples will serve to further amplify the invention.

EXAMPLE 1 A steel alloy made from electrolytic iron alloyed with 0.01percent by weight of carbon and 0.5 percent by weight of silicon wasmelted in a crucible assembly and thereafter held at a temperature of1550C. Under a head pressure provided by a gas mixture of argon andcarbon monoxide, the melt was streamed through a 6 mil beryllia orificeand thence into an atmosphere containing a mixture of carbon monoxideand helium. During streaming, the overhead carbon monoxide partialpressure was maintained at about 12 mm Hg (equilibrium for the meltsilica-carbon reaction). As the molten stream emerged from the orifice,this equilibrium value was exceeded by the applied partial pressure ofcarbon monoxide in the gas mixture immediately below the orifice. Thiscaused silica to precipitate and form an enveloping film about theperiphery of the extruded stream. The film-stabilized stream was thencaused to pass through a gas cooling tube where it solidified in theform of a fine diameter steel wire. In the course of prolonged streamingunder these conditions plugging of the orifice was not encountered.

EXAMPLE 2 A steel alloy made from electrolytic iron alloyed with 0.4percent by weight of carbon and 1.5 percent by weight of silicon wasmelted in a crucible assembly and thereafter held at a temperature of1515C. The melt was streamed through a 6 mil orifice to produce finediameter wire in accordance with the procedure of Example 1 above exceptfor the difference in the carbon monoxide pressure over the melt. Underthe conditions of this example equilibrium for the melt silica-carbonreaction is 230 mm of Hg and the overhead partial pressure of carbonmonoxide was maintained at substantially that value. As in Example 1, noorifice plugging was encountered while streaming.

EXAMPLE 3 A sample of commercial steel having a carbon content of 0.2percent by weight was alloyed with 1.5 percent by weight of silicon.This steel alloy was brought to the melt in a crucible assembly andthereafter held at a temperature of 1525C. The melt was extruded througha 6 mil orifice to produce fine diameter wire as in Example 1, above.However, in contrast to Example 1, the equilibrium partial pressure ofcarbon monoxide for the silica-carbon reaction within the melt, asdetermined from the concentrations of silicon and carbon and the melttemperature, is mm of Hg. Thus, the partial pressure of carbon monoxideabove the melt was maintained at approximately this value. Again, therewas no evidence of orifice plugging during the course of extrusion.

While there has been described what presently are considered to be thepreferred embodiments of this invention, it will be apparent to thoseskilled in the art that various changes and modififications may be madewithout departing from the invention. It is to be understood, therefore,that the invention is limited only by a.

proper construction of the language in the claims which follow.

I claim:

1. In a method for producing fine diameter wire from the melt of asteel-silicon alloy wherein said melt is extruded through an orifice asa continuous molten stream and into an oxygen-containing atmospherewhere a solid film of silica is caused to form about the surface of thestream to preserve its continuity until solidified, the improvementwhich comprises:

a. heating to the melt an alloy comprised of steel and at least 0.5percent by weight of silicon;

b. maintaining a pressurized gas mixture over the melt consisting of aninert gas and an oxygencontaining gas;

c. controlling the oxygen potential of the melt by means of saidoxygen-containing gas to a level wherein the activity of silica withinthe melt is maintained at from 0.3 to unity;

d. causing said melt to extrude through an orifice as a continuousmolten stream and into an oxygencontaining gaseous atmosphere having thecapacity for increasing the oxygen potential of said stream to a levelwherein silica is caused to precipitate and form a stabilizing filmabout the periphery of said stream; e. cooling said film-stabilizedmolten stream to the solid state.

2. The method of claim 1, wherein said steel-silicon melt contains fromabout 0.01 to 4.3 percent by weight of carbon and from about 0.5 to 5.0percent by weight of silicon.

3. The method of claim 1, wherein said gas mixture over the meltconsists of an inert gas and a gas selected from the group consisting ofcarbon monoxide, carbon dioxide, oxygen and steam.

4. The method of claim 3, wherein said inert gas is argon.

5. The method of claim 1, wherein said melt is extruded as a moltenstream into a gaseous atmosphere selected from the group consisting ofcarbon monoxide, carbon dioxide, oxygen, sulfur dioxide and steam.

6. The method of claim 1, wherein the oxygen containing gas in the gasmixture over the melt is carbon monoxide and the melt is extruded as amolten stream into a gaseous atmosphere of carbon monoxide.

7. The method of claim 1, wherein the oxygen potential of the melt iscontrolled to where the activity of silica is from 0.9 to unity.

8. The method of claim 1, wherein hydrogen or helium are employed as acooling gas to cool said filmstabilized molten stream to the solidstate.

1. IN A METHOD FOR PRODUCING FINE DIAMETER WIRE FROM THE MELT OF ASTEEL-SILICON ALLOY WHEREIN SAID MELT IS EXTRUDED THROUGH AN ORIFICE ASA CONTINUOUS MOLTEN STREAM AND INTO AN OXYGEN-CONTAINING ATMOSPHEREWHERE A SOLID FILM OF SILICA IS CAUSED TO FORM ABOUT THE SURFACE OF THESTREAM TO PRESERVE ITS CONTINUITY UNTIL SOLIDIFIED, THE IMPROVEMENTWHICH COMPRISES: A. HEATING TO THE MELT AN ALLOY COMPRISED OF STEEL ANDAT LEAST 0.5 PERCENT BY WEIGHT OF SILICON; B. MAINTAINING A PRESSURIZEDGAS MIXTURE OVER THE MELT CONSISTING OF AN INERT GAS AND ANOXYGEN-CONTAINING GAS; C. CONTROLLING THE OXYGEN POTENTIAL OF THE MELTBY MEANS OF SAID OXYGEN-CONTAINING GAS TO A LEVEL WHEREIN THE ACTIVITYOF SILICA WITHIN THE MELT IS MAINTAINED AT FROM 0.3 TO UNITY; D. CAUSINGSAID MELT TO EXTRUDE THROUGH AN ORIFICE AS A CONTINUOUS MOLTEN STREAMAND INTO AN OXYGEN-CONTAINING GASEOUS ATMOSPHERE HAVING THE CAPACITY FORINCREASING THE OXYGEN POTENTIAL OF SAID STREAM TO A LEVEL WHEREIN SILICAIS CAUSED TO PRECIPITATE AND FORM A STABILIZING FILM ABOUT THE PERIPHERYOF SAID STREAM; E. COOLING SAID FILM-STABILIZED MOLTEN STREAM TO THESOLID STATE.
 2. The method of claim 1, wherein said steel-silicon meltcontains from about 0.01 to 4.3 percent by weight of carbon and fromabout 0.5 to 5.0 percent by weight of silicon.
 3. The method of claim 1,wherein said gas mixture over the melt consists of an inert gas and agas selected from the group consisting of carbon monoxide, carbondioxide, oxygen and steam.
 4. The method of claim 3, wherein said inertgas is argon.
 5. The method of claim 1, wherein said melt is extruded asa molten stream into a gaseous atmosphere selected from the groupconsisting of carbon monoxide, carbon dioxide, oxygen, sulfur dioxideand steam.
 6. The method of claim 1, wherein the oxygen containing gasin the gas mixture over the melt is carbon monoxide and the melt isextruded as a molten stream into a gaseous atmosphere of carbonmonoxide.
 7. The method of claim 1, wherein the oxygen potential of themelt is controlled to where the activity of silica is from 0.9 to unity.8. The method of claim 1, wherein hydrogen or helium are employed as acooling gas to cool said Film-stabilized molten stream to the solidstate.